A few times over the last few week or so I've come on to the MoT forum only to find that I've been logged out and have to log in again. It's no big deal as it's only happened a few times but I wondered why it's happening.

I get this sometimes.
Do you tick the little box saying 'keep me signed in'?
Also, (having just perused the FAQ section that we now have a nice link to) there is a similar box within the User Control Panel which you can check/uncheck. Dunno if that will help.